Renewable energies take up more percentages among the power energy structure than ever before with the rapid development of the renewable energies integration technologies. It has been a great challenge that how to ensure a balance between energy supply and demand under this circumstance. Voltage source converter (VSC) based high-voltage direct-current (HVDC) grids are considered as an important way to integrate renewable energies. When integrated through VSC based HVDC grids, those converters at the renewable energies source base are called sending-end converters, whereas those converters connected to the main AC grids are called receiving-end converters. The power supplied by the renewable energies will be transmitted to the main AC grids through the VSC based HVDC grids.
DC voltage serves as an indicator of power balance in the HVDC grids. When the power supplied to the HVDC grids surpasses the demanded, the DC voltage increases, otherwise it decreases. Consequently, DC voltage in HVDC grids has similar characteristics to the frequency in the AC grids, both of which are the measure of power balance in the network. Apparently, it is a primary task to maintain a good control of the DC voltage in HVDC grids.
System level DC voltage control methods of VSC based HVDC grids reported in prior art, mainly include: 1) master-slaver control method; 2) DC voltage margin control method; and 3) DC voltage droop control method.
1) Master-slaver control method. The constant DC voltage controlled station is called master station under normal operating conditions, whereas the other stations equipped with backup DC voltage control are called slaver stations. This control method requires telecommunication between master and slaver stations. When the master station is out of service, a backup DC voltage control signal will be communicated to the slaver station via the protection system, so as to maintain a stable operation of the HVDC grids. The main problem of the master-slaver control method is that the slaver station cannot take over the function of DC voltage control smoothly.
2) DC voltage margin control method. In order to overcome the problem encountered by master-slaver control, DC voltage margin control is proposed to allow other stations to take the role of DC voltage control without telecommunication. The main idea of the DC voltage margin control is to set up a backup constant DC voltage controlled station, whose DC voltage reference is different from that of the current master station. Once the current master station breaks down, the increasing or decreasing DC voltage automatically triggers the margin DC voltage control in the backup station without communication.
3) DC voltage droop control method. Both active power and DC voltage are controlled by the DC voltage droop controller. The mission of regulating the DC voltage is distributed to several stations so as to share the power imbalance simultaneously under disturbances.
However, the main problems of DC voltage margin control method are as follows: firstly, since only one converter station participates in power sharing under disturbances, it takes more time to regain the stable operation compared to DC voltage droop control method; secondly, it is difficult to set up and coordinate the DC voltage reference especially when there are more than one backup constant DC voltage controlled stations in the HVDC grids. The main drawback of DC voltage droop control method is that the power transferred by the droop-controlled station cannot be controlled precisely.
In a word, the drawbacks mentioned above keep these common system level DC voltage control strategies from being implemented further to a certain extent.